As originally proposed, if American Tobacco had been permitted to merge with Brown & Williamson, which is based in Louisville, Ky., the combined company would have had an 18 percent share of the United States cigarette market, compared with American Tobacco's current share of 11 percent. The Philip Morris Companies, the biggest cigarette company, has a 44 percent share, and the RJR Nabisco Holdings Corporation has 27 percent.