  Charge transport in topological insulators is primarily characterised by
so-called topologically projected helical edge states, where charge carriers
are correlated in spin and momentum. In principle, dissipation-less current can
be carried by these edge states as backscattering from impurities and defects
is suppressed as long as time-reversal symmetry is not broken. However, applied
magnetic fields or underlying nuclear spin-defects in the substrate can break
this time reversal symmetry. In particular, magnetic impurities lead to
back-scattering by spin-flip processes. We have investigated the effects of
point-wise magnetic impurities on the transport properties of helical edge
states in the BHZ model using the Non-Equilibrium Green's Function formalism
and compared the results to a semi-analytic approach. Using these techniques we
study the influence of impurity strength and spin impurity polarization. We
observe a secondary effect of defect-defect interaction that depends on the
underlying material parameters which introduces a non-monotonic response of the
conductance to defect density. This in turn suggests a qualitative difference
in magneto-transport signatures in the dilute and high density spin impurity
limits.
