  We develop a theory of the conductance of a quantum dot connected to two
leads by single-mode quantum point contacts. If the contacts are in the regime
of perfect transmission, the conductance shows no Coulomb blockade oscillations
as a function of the gate voltage. In the presence of small reflection in both
contacts, the conductance develops small Coulomb blockade oscillations. As the
temperature of the system is lowered, the amplitude of the oscillations grows,
and eventually sharp periodic peaks in conductance are formed. Away from the
centers of the peaks the conductance vanishes at low temperatures as $T^2$, in
agreement with the theory of inelastic co-tunneling developed for the
weak-tunneling case. Conductance near the center of a peak can be studied using
an analogy with the multichannel Kondo problem. In the case of symmetric
barriers, the peak conductance at $T\to 0$ is of the order of $e^2/\hbar$. In
the asymmetric case, the peak conductance vanishes linearly in temperature.
