In a two-way circuit, crosstalk problems are most noticeable when the signal leaves the transmitter. At this point the detector for the adjacent pair is closest and the energy level is highest. The resistance to the flow of current is expressed in ohms. Every material offers some resistance or opposition to the flow of electric current. The size and type of material in a circuit is chosen to provide the lowest resistance possible.
Copper is an excellent conductor for the price.
The above technical reference material is provided by Coleman Cable.