  We study the electron transport in open quantum-dot systems described by the
interacting resonant-level models with Coulomb interactions. We consider the
situation in which the quantum dot is connected to the left and right leads
asymmetrically. We exactly construct many-electron scattering eigenstates for
the two-lead system, where two-body bound states appear as a consequence of
one-body resonances and the Coulomb interactions. By using an extension of the
Landauer formula, we calculate the average electric current for the system
under bias voltages in the first order of the interaction parameters. Through a
renormalization-group technique, we arrive at the universal electric current,
where we observe the suppression of the electric current for large bias
voltages, i.e., negative differential conductance. We find that the suppressed
electric current is restored by the asymmetry of the system parameters.
