  Nonreciprocal transport in uniform systems has attracted great research
interest recently and the existing theories mainly focus on the diffusive
regime. In this study, we uncover a novel scenario for nonreciprocal charge
transport in the ballistic regime enabled by asymmetric band structures of the
system. The asymmetry of the bands induces unequal Coulomb potentials within
the system as the bias voltage imposed by the electrodes inverts its sign. As a
result, the bands undergo different energy shifts as the current flows in
opposite directions, giving rise to the nonreciprocity. Utilizing the
gauge-invariant nonlinear transport theory, we show that the nonreciprocal
transport predominantly originates from the second-order conductance, which
violates the Onsager reciprocal relation but fulfills a generalized reciprocal
relation similar to that of unidirectional magnetoresistance. The ballistic
nonreciprocal transport phenomena differ from the diffusive ones by considering
the internal asymmetric Coulomb potential, a factor not accounted for in
diffusive cases but undeniably crucial in ballistic scenarios. Our work opens a
avenue for implementing nonreciprocal transport in the ballistic regime and
provides an alternative perspective for further experimental explorations for
nonreciprocal transport.
