Slow Down the ball speed in every collesion

I am working on simple Brick game.I have ball which is basically sephere.I am using using Phyiscs Regid Body meterial.The problem is when my ball hits the wall its speed gradually slow down.Mean after every collesion with wall or padal its speed will slow down this is the simple c# script of my ball game.How to over come this. using UnityEngine; using System.Collections;

 public class BallScript : MonoBehaviour {
 
     // Use this for initialization
     void Start () {
     rigidbody.AddForce(500f,500f,0);
     }
     
     // Update is called once per frame
     void Update () {
     //rigidbody.AddForce(10f,100f,0);
     }
 }