Slowly reduce the speed of bouncing Sphere

I have a sphere that is bouncing continuously, I want to reduce the sphere speed such that its bounciness gradually reduces and finally comes to end. I m reducing my bounce value in FixedUpdate function as: bounce = 10; while(bounce <= 0) { bounce = bounce - bounce * Time.deltaTime; } and And In OncollisionEnter I m writting: void OnCollisionEnter(Collision col) { if(col.gameObject.tag == “Walls”) { rigidbody.velocity = new Vector3(0, 0, bounce); } }
My Sphere, physics material is set as: Dynamic Friction:0.6, Static Friction: 0.6, Bounciness:1, Friction Combine: Average, Bounce Combine: Maximim. Along with this My rigidbody which is added to Sphere have following details: Mass: 0.5, Drag: 0.99, Angular Drag: 0.05

Create your own physic materials and apply them to the “Walls” and the ball…

http://docs.unity3d.com/Documentation/Components/class-PhysicMaterial.html