Combine thawed orange juice concentrate in a bowl with the marjoram, garlic powder and nutmeg. Dip each chicken breast half 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 6-8 hours, or on high for about 4 hours. 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; delicious with brown or white rice.