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; saut 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.