Bouncy Ball stops without enough force

I am building a 3D Pong clone and want to implement a frictionless bouncing ball.

These are the steps i took:

1. Create a box in which the ball can bounce, consisting of standard cubic objects.

2. Create a ball using a spherical object

3. Add a Rigidbody to said ball (Mass: 1, Drag: 0, A.Drag: 0, no Gravity)

4. Add a bouncy Physics Material to the balls Sphere Collider (D.Friction: 0, S.Friction: 0, Bounciness: 1, Friction Combine: min, Bounce Combine: max)

5. Add a script to the ball that adds force to the ball on start.

Script Snippet:

 void Start() {
     this.getComponent<RigidBody>().addForce(0, 0, 100);
 }

When i play the scene, the ball will fly towards the wall and then stop. If i change the added force to (0,0,101) it works as intended. If i lower the mass of the ball and use the old force (0,0,100) it also works. But it will always stop at the wall, if the added force is <= mass*100.

What am i missing?