  The discovery of the Quantum Spin Hall state, and topological insulators in
general, has sparked strong experimental efforts. Transport studies of the
Quantum Spin Hall state confirmed the presence of edge states, showed ballistic
edge transport in micron-sized samples and demonstrated the spin polarization
of the helical edge states. While these experiments have confirmed the broad
theoretical model, the properties of the QSH edge states have not yet been
investigated on a local scale.
  Using Scanning Gate Microscopy to perturb the QSH edge states on a sub-micron
scale, we identify well-localized scattering sites which likely limit the
expected non-dissipative transport in the helical edge channels. In the
micron-sized regions between the scattering sites, the edge states appear to
propagate unperturbed as expected for an ideal QSH system and are found to be
robust against weak induced potential fluctuations.
