  To understand the seemingly absent temperature dependence in the conductance
of two-dimensional topological insulator edge states, we perform a numerical
study which identifies the quantitative influence of the combined effect of
dephasing and elastic scattering in charge puddles close to the edges. We show
that this mechanism may be responsible for the experimental signatures in
HgTe/CdTe quantum wells if the puddles in the samples are large and weakly
coupled to the sample edges. We propose experiments on artificial puddles which
allow to verify this hypothesis and to extract the real dephasing time scale
using our predictions. In addition, we present a new method to include the
effect of dephasing in wave-packet-time-evolution algorithms.
