Another idea is the curve looks to be an arc on a circle. Here's one idea to constrain the ball to that arc:
https://dl.dropboxusercontent.com/u/542 ... pprox.capx
To make it perfect you'd need to tweak stuff. The radius, lowAng, highAng, the angle of the track and the origin of the track.
i want this link , can you or anyone please reupload this file ? thanks