  Due to time-reversal symmetry (TRS), two dimensional topological insulators
support counter-propagating helical edge modes. Here we show that, unlike the
infinitely sharp edge potential utilized in traditional calculations, an
experimentally more realistic smooth edge potential gives rise to edge
reconstruction and, consequently, spontaneous TRS breaking. Such edge
reconstruction may lead to breaking of the expected perfect conductance
quantization, to a finite Hall resistance at zero magnetic field, and to a
likely spin current. This calculation underpins the fragility of the
topological protection in realistic systems, which is of crucial importance in
proposed applications.

Strongly Correlated Electrons;Mesoscale and Nanoscale Physics