  The zero gap surface states of a 3D-topological insulator host highly mobile
Dirac fermions with spin locked to the momentum. The high mobility attributed
to absence of back scattering is reduced in presence of impurities on the
surface. In particular, we discuss and compare scattering times for localized
impurities on the surface, scattering between states of opposite helicity
located on different surfaces coupled through a hybridization potential, and
the role of magnetic impurities. Magnetic impurities give rise to an additional
spin suppression factor. The role of warped bands and its influence on
topological factors that can enhance the overall surface mobility is examined.
