  We study the temperature dependence ($77$ K - $475$ K) of the longitudinal
resistivity and Hall coefficient of thin films (thickness $20$ nm) of three
dimensional topological Dirac semimetal Na$_3$Bi grown via molecular beam
epitaxy (MBE). The temperature-dependent Hall coefficient is electron-like at
low temperature, but transitions to hole-like transport around $200$ K. We
develop a model of a Dirac band with electron-hole asymmetry in Fermi velocity
and mobility (assumed proportional to the square of Fermi velocity) which
explains well the magnitude and temperature dependence of the Hall resistivity.
We find that the hole mobility is about $7$ times larger than the electron
mobility. In addition, we find that the electron mobility decreases
significantly with increasing temperature, suggesting electron-phonon
scattering strongly limits the room temperature mobility.
