The ballistic transport properties of Cu nanowires (NWs) under electric fields are investigated using first-principles density function theory for future application as interconnects in microelectronics. It is found that as electric field strength increases, the amount of quantum conduction of a nonhelical atomic strand decreases, while that of a helical atomic strand is in an opposite tendency. The changes are decided by the changes of atomic layer distance of the NWs and the related electronic distribution along the axis of the NWs.