Combine thawed orange juice concentrate (not regular orange juice) in bowl along with the marjoram, garlic powder and nutmeg. Split the chicken breasts to make 6 serving sizes. Dip each piece into the orange juice to coat completely. Place in crockpot. Pour the remaining orange juice mixture over the chicken. Cover and cook on low for 7-9 hours, or cook on high for 4 hours if you wish. Precise cooking time is not important in crockpot cooking.
When chicken is done, remove to serving platter. Pour the sauce that remains in crockpot into a saucepan. Mix the cornstarch and water and stir into the juice in pan. Cook over medium heat, stirring constantly, until thick and bubbly. Serve the sauce over the chicken.