We investigate quantum transport via surface states in a nanostep junction on
the surface of a 3D topological insulator that involves two different side
surfaces. We calculate the conductance across the junction within the
scattering matrix formalism and find that as the bias voltage is increased, the
conductance of the nanostep junction is suppressed by a universal factor of 1/3
compared to the conductance of a similar planar junction based on a single
surface of a topological insulator. We also calculate and analyze the Fano
factor of the nanostep junction and predict that the Fano factor saturates at
1/5, five times smaller than for a Poisson process