angle-resolved photoemission. The electron-phonon coupling strength is determined in a spectral region for
which only intraband scattering involving the topological surface band is possible. The electron-phonon coupling
constant is found to be λ = 0.25(5), more than an order of magnitude higher than the corresponding value for
intraband scattering in the noble-metal surface states. The stability of the topological state with respect to surface
irregularities was also tested by introducing a small concentration of surface defects via ion bombardment. It is
found that, in contrast to the bulk states, the topological state can no longer be observed in the photoemission
spectra, and this cannot merely be attributed to surface defect-induced momentum broadening.