conductor and its properties
OK, so you are asking why lower conductivity means a higher relaxation time. You expected it to be the other way around.

Picture yourself trying to get across a crowded room. If everyone is holding still, you will be able to get across quickly. If everyone is moving around, you'll get bounced around and have trouble getting across. 

This is also true if you happen to be an electron. Conductivity is better if all the other electrons are holding still. So, this is why lower temperature leads to higher current.

Relaxation time is better if all the electrons aren't bouncing all over the place--they can all walk back to their "places" more quickly because they're not interfering so much with each other. (And you can think of impurities etc that would increase relaxation time as a bunch of furniture in the room that they have to move around).