Preheat oven to 350°.
Combine sugar and butter in a bowl; beat at medium speed of a mixer until well-blended. Add banana, milk, sour cream, and egg whites; beat well, and set aside.
Combine flour, baking soda, and salt; stir well. Add dry ingredients to creamed mixture, beating until blended.
Spoon batter into 4 (5 x 2 1/2-inch) miniature loaf pans coated with cooking spray. Bake at 350° for 45 minutes or until a wooden pick inserted in center comes out clean. Let cool in pans 10 minutes on a wire rack; remove from pans. Let cool completely on wire racks.
Note: To make one 9-inch loaf, spoon batter into a 9 x 5-inch loaf pan coated with cooking spray; bake at 350º for 1 hour and 10 minutes. Yield: 1 loaf, 20 servings (serving size: 1 slice).