The present thesis deals with the properties of nanoscale conductors which are currently under intense research. These systems are challenging objects of basic research due to the appearance of many-body quantum effects and their nonequilibrium properties. Besides the academic interest, the possibility of technical applications motivates to study these systems as well. Different physical scenarios are presented in this work. This includes a nonequilibrium quantum transport investigation as well as the study of strongly correlated electrons in single and double quantum dots. Furthermore, the influence of a dissipative environment on quantum coherence is examined.
For nonequilibrium quantum transport we have developed a numerical method which allows to calculate path integrals of interacting fermionic systems. The scheme is based on a deterministic iterative summation of the path integral for the generating function of the nonequilibrium current. Self-energies due to the leads, being non-local in time, are fully taken into account within a finite memory time, thereby including non-Markovian effects. Our numerical results are extrapolated both to vanishing time discretization and to infinite memory time. This extrapolation scheme converges except at very low temperatures, and the results are then numerically exact. The method is applied to nonequilibrium transport through an Anderson dot.