Bouncing balls with collision from any 2Dcolliders/wall can be achieved by putting continuous force to the game object and assign a physics material to the game object you like to bounce on collision.
1. Gameobject must have rigidbody attached to it with 0 gravity.
2. Gameobject must have a 2D collider with a 2d physics Material having 0 friction and 0.8 bounce.
3. Assign a script name BouncingObjects having the following line of code in it.
 

using UnityEngine;
using System.Collections;

public class BouncingObjects : MonoBehaviour {

    
    float x,y;
    public float Speed=1f;//can be vary manually
    // Use this for initialization
    void Start ()
    {

        x = Random.Range (-1, 2);
        y = Random.Range (-1, 2);
    }    
//    continously assigning x and y.
    void FixedUpdate ()
    {
        
        this.transform.GetComponent<Rigidbody2D> ().AddForce(new Vector2 (x,y)*Speed,ForceMode2D.Force);
    }

//Changing x and y on collision
    void OnCollisionEnter2D(Collision2D coll)
    {

        if (x < 0)        
            x = 1;
        else if (x > 0)
            x = -1;
        else
            x = 1;

        if (y < 0)        
            y = 1;
        else if (y > 0)
            y = -1;
        else
            y = -1;
            
        }
}