  Non-equilibrium transport through a quantum dot with one spin-split
single-particle level is studied in the cotunneling regime at low temperatures.
The Coulomb diamond can be subdivided into parts differing in at least one of
two respects: what kind of tunneling processes (i) determine the
single-particle occupations and (ii) mainly contribute to the current. No
finite systematic perturbation expansion of the occupations and the current can
be found that is valid within the entire Coulomb diamond. We therefore
construct a non-systematic solution, which is physically correct and
perturbative in the whole cotunneling regime, while smoothly crossing-over
between the different regions. With this solution the impact of an intrinsic
spin-flip relaxation on the transport is investigated. We focus on peaks in the
differential conductance that mark the onset of cotunneling-mediated sequential
transport. It is shown that these peaks are maximally pronounced at a
relaxation roughly as fast as sequential tunneling. The approach as well as the
presented results can be generalized to quantum dots with few levels.
