  We present direct imaging measurements of charge transport across a 1
cm$\times$ 1 cm$\times$ 4 mm crystal of high purity silicon ($\sim$20
k$\Omega$cm) at temperatures between 500 mK and and 5 K. We use these data to
determine the intervalley scattering rate of electrons as a function of the
electric field applied along the $\langle 111 \rangle$ crystal axis, and we
present a phenomenological model of intervalley scattering that explains the
constant scattering rate seen at low-voltage for cryogenic temperatures. We
also demonstrate direct imaging measurements of effective hole mass anisotropy,
which is strongly dependent on both temperature and electric field strength.
The observed effects can be explained by a warping of the valence bands for
carrier energies near the spin-orbit splitting energy in silicon.
