In the first part of this thesis a novel Eikonal approach to calculate the dephasing time in disordered conductors is presented. The applied method has, compared to earlier calculations, the advantage that it does not require to introduce any cutoffs and is thus free of ambiguities. It is applied to study the influence of electron-electron interactions in a semiclassical limit as well as dephasing due to external fields. Regarding external fields, the influence of a finite wave-length q0 ist investigated and a new effect is found. The finding is in agreement with so far unexplained experimetnal data. The developed method is also used to examine the Diffuson propagator in the presence of dephasing fields. In the second part of this work the time-dependent non-equilibrium current in a mesoscopic metral ring threaded by a static magnetic flux Phi that ist generated by a time-dependent electric field oscillating with frequency Omega is investigated. It is shown that in quadratic order in the field there are three fundamentally different contributions to the current. (a) A time-independent contribution which can be obtained from a thermodynamic derivative. (b) A term increasing linearly in time that can be understood in terms of Fermi's golden rule. The derivation of this term requires a careful treatment of the infinitesimal imaginary parts that are added to the real frequecny Omega when the electric field is adiabatically switched on. (c) Finally, there is also a time-dependent current oscillating with frequency 2Omega. Extensive numerical results are presented to underline the theoretical findings.
