  Quantum spin Hall devices with edges much longer than several microns do not
display ballistic transport: that is, their measured conductances are much less
than $e^2/h$ per edge. We imaged edge currents in InAs/GaSb quantum wells with
long edges and determined an effective edge resistance. Surprisingly, although
the effective edge resistance is much greater than $h/e^2$, it is independent
of temperature up to 30 K within experimental resolution. Known candidate
scattering mechanisms do not explain our observation of an effective edge
resistance that is large yet temperature-independent.
