Say we have a solid wire made of Cu. It seems most electrons transport at the surface of the wire. Right? So is there any difference of the resistance if we have a hollow Cu wire (same diameter with the solid wire). I think the answer shoule be "yes". Right? Becasuse, the resistance R=p*L/A (p is the resistivity). But if the electron transport are only in the surface of the wires at both cases(solid and hollow wires) , why there is difference of the resistance?