Not pure copper, they need to be tight enough for minimal sag between supports, and not wear from abrasion (the phantograph touching the wire). Copper doesn't have good characteristics for that. I don't know what Caltran uses, but it is common to have a steel core and plate it with something else.
Quite to the contrary, copper is the usual material used (either pure, or slightly alloyed to improve tensile strength and abrasion behaviour and things like that without compromising too much on copper's superior electrical conductivity, though that extra tensile strength is only really important for high speed railways – conventional railways could perfectly get by on pure copper alone). Sag is solved by tensioning the wire and using a catenary wire for additional support between poles, and eventual abrasion of the wire is just a fact of life for electric railways.
