RJR Nabisco Inc. has broken off buyout talks with Kohlberg Kravis Roberts & Co., raising the possibility that the powerful investment firm might launch a hostile $20.3 billion tender offer for the food and tobacco conglomerate. The failure of the talks appeared to set the stage for a showdown between Kohlberg Kravis and Shearson Lehman Hutton Inc., Nabisco's financial partner, over what would be the nation's biggest corporate acquisition. After a meeting of Nabisco's outside directors Wednesday, President F. Ross Johnson said the group concluded Kohlberg Kravis's proposal to acquire Nabisco for $90 a share was not in the company's best interest.