It is well known that back-scattering of surface electrons in topological insulators is prohibited due to Kramer's degeneracy theorem as long as Time Reversal Symmetry is not broken by magnetic field or magnetic impurities. I would like to know what effect this has on scattering length and scattering times of a topological insulator. Should they not be very large in absence of magnetic impurities; the logic being that electrons will not be scattered. Or even in the case when back-scattering is absent, can electrons still be scattered, for example by angles not equal to 180 °.