Preheat oven to 350°F. Coat a 2-quart square baking dish with nonstick coating; set aside. Cook macaroni as directed on package. Drain and return to pot; keep warm.
While macaroni cooks, melt light butter in a medium saucepan. Add flour and salt; stir for 1 minute. Add milk; stir over medium heat until thickened and bubbly. Remove from heat; stir in cheese until melted.
Toss together macaroni, sauce, chicken breast and barbeque sauce. Transfer to prepared dish. Sprinkle evenly with breadcrumbs. Bake for 20 minutes or until heated through.