  Crystals with symmetry-protected topological order, such as topological
insulators, promise coherent spin and charge transport phenomena even in the
presence of disorder at room temperature. Still, a major obstacle towards an
application of topological surface states in integrated circuits is a clear,
reliable, and straightforward read-out independent of a prevailing charge
carrier density in the bulk. Here, we demonstrate how to image and read-out the
local conductance of helical surface modes in the prototypical topological
insulators Bi2Se3 and BiSbTe3. We apply the so-called Shockley-Ramo theorem to
design an optoelectronic probe circuit for the gapless surface states, and
surprisingly find a precise conductance quantization at 1e2/h. The
unprecedented response is a clear signature of local spin-polarized transport,
and it can be switched on and off via an electrostatic field effect. The
macroscopic, global read-out scheme is based on the displacement current
resistivity, and it does not require coherent transport between electrodes.6 It
provides a generalizable platform for studying further non-trivial gapless
systems such as Weyl-semimetals and quantum spin-Hall insulators.
