We present a full quantum transport study of the zero-temperature coefficient (ZTC) point for sub-10 nm gateall-around nanowire p-type field effect transistors (GAA NW pFETs). The phonon scattering effects are included through the self-consistent Born approximation in the non-equilibrium Green's function framework. The main findings are that the ZTC point can be present in GAA NW pFETs in sub-10 nm regime and the gate voltage at the ZTC point shows an opposite trend and has an upper limit at a certain gate length. This is due to the interplay between the ballisticity ratio and the ballistic current ratio, which can be explained only by the quantum transport simulations.