  Absence of backscattering and occurrence of weak anti-localization are two
characteristic features of topological insulators. We find that the
introduction of defects results in the appearance of a negative contribution to
magnetoresistance in the topological insulator BiSbTeSe2, at temperatures below
50 K. Our analysis shows that the negative magnetoresistance originates from an
increase in the density of defect states created by introduction of disorder,
which leaves the surface states unaffected. We find a decrease in the magnitude
of the negative magnetoresistance contribution with increasing temperature and
a robustness of the topological surface states to external disorder.
