Preheat oven to 425 degrees. Lightly spray a cookie sheet with nonfat cooking spray. Combine bread crumbs, chili powder, cumin and salt on waxed paper. Cut potatoes lengthwise into quarters. Scoop out potato pulp, leaving 1/4 inch shells. Brush shells with mayonnaise, then roll in the crumb mixture to coat. Bake on cookie sheet for 25 minutes. Turn over and bake 25 minutes longer, or until crisp.