  Within the framework of Boltzmann equation, we present a $\mathbf{k\cdot p}$
theory based study for the low-field mobilities of InSb nanowires (InSb NWs)
with relatively large cross sectional sizes (with diameters up to 51.8 nm). For
such type of large size nanowires, the intersubband electron-phonon scattering
is of crucial importance to affect the scattering rate and then the mobility.
In our simulation, the lowest 15 electron subbands and 50 transverse modes of
phonons are carefully accounted for. We find that, up to the 51.84 nm diameter,
the mobility monotonously increases with the diameter, not yet showing any
saturated behavior. We also find that, while the bulk InSb mobility is
considerably higher than the bulk Si, the small size (e.g. $\sim 3$ nm
diameter) nanowires from both materials have similar magnitude of mobilities.
This implies, importantly, that the mobility of the InSb NWs would decrease
faster than the SiNWs as we reduce the cross sectional size of the nanowires.
