At data rates, transmission lines are used in copper. This is why CAT-5e has higher requirements on the twisted pairs than CAT-3 (or plain old phone line). 66%

At data rates, transmission lines are used in copper. This is why CAT-5e has higher requirements on the twisted pairs than CAT-3 (or plain old phone line). 66% turns out to be a surprisingly consistent approximation for both copper and fiber.