Miss Heidi Vallo Mitchell, 20, in solid gold halter draped with casual weight under her white wool jacket, says hello in the doorway of her hotel room. She is holding a dozen long-stemmed roses and she has the kind of platinum glow that is acquired only by 20 years of growing up in California.

This is not the kind of look you associate with debutante balls, but that's what she's in from California for: last night's National Debutante Cotillion and Thanksgiving Ball, which is her fifth debut in two years.

She also has a press agent. The agent describes Heidi as "one of the top young glamor girls in Los Angeles," says he also represents "glamor girls such as Angie Dickinson and Stella Stevens," and points out that "all three are born under Libra, the charm sign of the zodiac."

Now Heidi's in Washington.

"We don't have that many balls in California," she says, once she's back inside her room with her mother, Ingrid, and her escort, Michael Hammer, grandson of the Occidental Petroleum tycoon Armand Hammer.

"We have them, but they're not as elaborate," says the mother, explaining that elaborate means about $5,000 apiece, for dress, airplane tickets and so on. "I always say, four days, $5,000."

"On the East Coast the girls are more sophisticated, they dress different, they study different subjects," says Heidi. "They don't study communications as much here. (She's majoring in broadcasting at Pepperdine University, in Malibu, where she is also president of her sorority, Theta Alpha Phi.) It seems to me the girls here are more grown-up than they are in California."

There was a ball in Palos Verdes, her home town, in 1979, and then she was one of two girls chosen from that ball to be a "coronet deb" in Los Angeles, in 1980. "It kind of depends on who you know," she says. That summer she went to Vienna for the Bal der Rosenkavaliers, and then there was the New Year's Eve Ball See DEBUTANTE, D4, Col. 4 DEBUTANTE, From D1 in New York.

She's also been "in about 15 beauty contests. I was Miss Junior Minnesota, I was Miss United California Teenager, I was Miss Palos Verdes, I was Miss Student Beauty of Los Angeles -- I got a $1,000 scholarship for that one."

This is her last deb party as a deb, as opposed to an ordinary guest, says her father, Paul Mitchell, a mammoth fellow who captained the 1944 football team at the University of Minnesota, later played professionally for the old New York Yankees and who is now in real estate and business. "This is the end -- now all she has to do is find a husband."

Everyone laughs, even Heidi's escort, Michael Hammer, donor of the ruby ring that Heidi says she's "testing this weekend to see if I like it." He swaps financial gossip with Mr. Mitchell, who then rolls out of the room saying: "I've gotta call my broker."

Mrs. Mitchell has been working on her daughter's behalf for years. "You get into the National Charity League," she says. "They like a certain type of mother, the workhorse type. I did charity work, I was in charge of a horse show and a thrift shop. Then your daughter gets in."

"You can be dropped if you don't show up for your charity jobs," says Heidi, who did 500 hours of work as a "Ticktocker." The name, she says, wagging a finger like a pendulum, "comes from the time going past so fast."

Her mother grew up in Minnesota. "There weren't many debs in my day. I don't think I knew what the word was."

Mr. Mitchell leans back inside the room. He says: "It's 106 7/8, up 1/8." He tells Heidi he just sold some of her stock. "I'm an amateur. Real estate is my business. I've been in it 20 years; '73, '74 was when it took off in Los Angeles. If you'd accumulated stuff and let it sit there, then you had it."

He is also vice president of what Heidi's press agent calls "the important Garrett Corporation," an aerospace company.

Heidi hopes to capitalize on her publicity -- she appeared in Liz Smith's gossip column in the New York Daily News wearing a leopard-patterned bathing suit. She wants to become a model.

"The people who make the most money in modeling are the ones who are known the best," she says.

"I used to model," says Mrs. Mitchell.

Heidi is asked about the gold halter.

"That's solid gold. We had to pay for it in two different checkbooks to hide it," says Mrs. Mitchell, glancing at the door from which Mr. Mitchell has again vanished.

"It was three months' allowance," says Heidi.

She is asked if she is Daddy's little girl.

"Uh-huh," she says.

"Is she ever," says her mother.