  One of the unique features of Dirac Fermions is pseudo-diffusive transport by
evanescent modes at low Fermi energies when the disorder is low. At higher
Fermi energies i.e. carrier densities, the electrical transport is diffusive in
nature and the propagation occurs via plane-waves. In this study, we report the
detection of such evanescent modes in the surface states of topological
insulator through 1/f noise. While signatures of pseudo-diffusive transport
have been seen experimentally in graphene, such behavior is yet to be observed
explicitly in any other system with a Dirac dispersion. To probe this, we have
studied 1/f noise in topological insulators as a function of gate-voltage, and
temperature. Our results show a non-monotonic behavior in 1=f noise as the
Fermi energy is varied, suggesting a crossover from pseudo-diffusive to
diffusive transport regime in mesoscopic topological insulators. The
temperature dependence of noise points towards conductance fluctuations from
quantum interference as the dominant source of the noise in these samples.
