The SBC Deal Shelved, What Tack For AT&T?
Analysis of AT&T's future following breakdown of merger talks with SBC Communications Inc; fact that talks were held at all seen as AT&T's acknowledgment that it is wary of competing on its own in deregulated market; company's peculiar handling of negotiations may have tarnished both its credibility and morale; financial logic of a merger is so compelling that many experts expect AT&T to try to resuscitate talks with SBC; failing that, AT&T could try to forge deal with another regional Bell company, like BellSouth, or with GTE; photo (M)