SADLY, Steve Cauthen, one of the great jockeys of the post-war era, has ridden his last race. Cauthen sits poised to announce his retirement from the saddle after failing to reach agreement on his lucrative riding contract with powerful owner Sheik Mohammed for 1993. ''I haven't made an absolute decision,'' Cauthen told me by phone from Kentucky. But with nobody likely to make an offer anywhere near that of the Sheik, and with no likelihood of h ''It was not as if I was asking for a rise,'' Cauthen told me. ''The terms offered were not acceptable and Sheik Mohammed's advisers were not willing to 'up' them. There is no animosity. The Sheik has been very good to me and I can understand his position. ''He's been making a lot of cutbacks lately and my position has something to do with that. I don't think he felt the disappointments of the past year were in any way my fault. But if there is one regret, it is that I believe we were unfortunate not to have better horses during my time with him.'' There are widely-held fears that Sheik Mohammed could be disenchanted by racing these days. Cauthen said: ''I think Sheik Mohammed is disappointed with racing in general. He has spent a lot of money and there hasbeen a big drop in horse prices over the past few years.'' It is not immediately known what Cauthen plans to do in retirement, although he has long harboured a