A major technicians' union and ABC remained at an impasse Tuesday, with each side reporting no negotiations scheduled on ABC's proposed four-year contract to succeed the one that expired March 31. Officials for ABC and the National Assn. of Broadcast Employees and Technicians (NABET) held two days of federally mediated meetings last week in New York but failed to reach agreement on proposed modifications to ABC's offer. The union, which says it represents 2,150 ABC employees, including workers at KABC-TV here, has not called for a strike vote yet. Its last ABC walkout, in 1977, lasted 20 weeks.