Remove roots, outer leaves, and tops from leeks. Rinse leeks under cold running water; cut into 2-inch julienne strips to measure 1 cup, and set aside.
Sprinkle chicken with salt and pepper. Melt margarine in a nonstick skillet over medium heat. Add chicken; sauté 4 minutes on each side or until browned. Add leeks; cook 3 minutes or until soft. Add wine and broth; bring to a boil. Cover, reduce heat, and simmer 10 minutes or until chicken is done, turning chicken once. Remove chicken from skillet; set aside, and keep warm. Add mushrooms to leek mixture in skillet; cook over high heat 1 minute. Add milk and tarragon; reduce heat, and simmer, uncovered, 3 minutes or until sauce is thick. Serve chicken and sauce over brown rice.