  We report electrical conductance measurements of Bi nanocontacts created by
repeated tip-surface indentation using a scanning tunneling microscope at
temperatures of 4 K and 300 K. As a function of the elongation of the
nanocontact we measure robust, tens of nanometers long plateaus of conductance
G0 = 2e^2/h at room temperature. This observation can be accounted for by the
mechanical exfoliation of a Bi(111) bilayer, a predicted QSH insulator, in the
retracing process following a tip-surface contact. The formation of the bilayer
is further supported by the additional observation of conductance steps below
G0 before break-up at both temperatures. Our finding provides the first
experimental evidence of the possibility of mechanical exfoliation of Bi
bilayers, of the existence of the QSH phase in a two-dimensional crystal, and,
most importantly, of the observation of the QSH phase at room temperature.
