  Universal conductance fluctuations (UCFs) are extracted in the
magnetoresistance responses in the bulk-insulating Bi2Te2Se microflakes. Its
two-dimensional character is demonstrated by the field-tilting
magnetoresistance measurements. Its origin from the surface electrons is
determined by the fact that the UCF amplitudes keep unchanged while applying an
in-plane field to suppress the coherence of bulk electrons. After considering
the ensemble average in a batch of micrometer-sized samples, the intrinsic UCF
magnitudes of over 0.37 e2/h is obtained. This agrees with the theoretical
prediction on topological surface states. All the evidence point to the
successful observation of the UCF of topological surface states.
