  The occurrence of two-particle inelastic backscattering has been conjectured
in helical edge states of topological insulators and is expected to alter
transport. In this Letter, by using a renormalization group approach, we
provide a microscopic derivation of this process, in the presence of a
time-reversal invariant Rashba impurity potential. Unlike previous approaches
to the problem, we are able to prove that such an effect only occurs in the
presence of electron-electron interactions. Furthermore, we find that the
linear conductance as a function of temperature exhibits a crossover between
two scaling behaviors: $T^{4K}$ for $K>1/2$ and $T^{8K-2}$ for $K<1/2$, with
$K$ the Luttinger parameter.
