Thomas Sheer, head of the FBI's New York office for 14 months, said he is retiring to become a counterterrorism consultant for a top international executive security firm. Sheer, 50, said he will step down on Jan. 1 from his $72,500-a-year job. He declined to say what he will earn in his new position. Sheer acknowledged that salary was a factor in his resignation, but he said that he is approaching the FBI's mandatory retirement age of 55 and wanted to seek new employment while he is more "marketable." In his new position, Sheer said that he will specialize in risk management and will advise the international company on industrial sabotage, drug problems, espionage, kidnaping and extortion. The New York FBI office has 1,200 agents and is the largest of the agency's 59 field offices.