This is what INS envisions with its new fencing scheme: About six miles of the tough new steel grid, concrete and chain-link fencing will replace old fencing on the border south of San Diego, at a point known as San Ysidro. A similar length will be erected in the downtown area and outskirts of El Paso, which sits on the international border. Congress last year appropriated $4 million for the El Paso and San Ysidro spans. Future construction would include another stretch at San Luis, Ariz., about 10 miles south of Yuma. Congress this year approved $1.5 million at the behest of Sen. Dennis DeConcini (D-Ariz.), who said the fence would protect Yuma from borderjumping thieves. In each instance, the new fence is intended to replace old barriers that are riddled with holes - some large enough to drive an automobile through.