From a discussion on Singlespeed - I discovered this lock.

Owners of the mini-Kryptonite Evolution 2000 (not the long shackle) you may have found me the solution, a sold-secure silver rated lock that's less than half the weight of any other. It would save me 2lb on my current lock, whilst still keeping the insurance company sweet... if it will fit the rack at work. Can anyone tell me the internal length and width of the shackle? Kryptonite only lists the overall dimensions.

Thanks Richard