The 20 richest women in the world got that way by inheriting their money, according to a study by the British society magazine Harpers and Queen. The only self-made billionaire on the magazine's list is Grete Schickedanz, 78, the owner of a German mail-order chain, whose $1.77 billion makes her the 10th richest woman in the world. Queen Elizabeth II again heads the list with a $13 billion fortune, followed by Johanna Quandt, widow of BMW car magnate ($5.1 billion), Imelda Marcos ($3 billion) and Atlanta sisters Anne Cox Chambers and Barbara Cox Anthony, heirs to a publishing fortune ($2.76 billion each). Other Americans on the list are No. 7 Jacqueline Mars Vogel, daughter of Mars Inc. candy magnate ($2.36 billion); No. 8 Alice L. Walton, daughter of discount retailer Sam Walton ($1.97 billion); No. 13 Margaret Cargill, daughter of grain trader ($1.28 billion); No. 16. Margaret Hunt Hill, 74, daughter of oil baron H.L. Hunt ($1.08 billion) and No. 19. Jean B. Kroc, widow of the McDonald's founder ($985 million).