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Coulomb-blockade phenomena and quantum fluctuations are studied in mesoscopic metallic tunnel junctions with high charging energies. If the resistance of the barriers is large compared to the quantum resistance, transport can be described by sequential tunneling. Here we study the influence of quantum fluctuations. They are strong when the resistance is small or the temperature is very low. A real-time approach is developed that allows the diagrammatic classification of resonant tunneling processes where different electrons tunnel coherently back and forth between the leads and the metallic island. With the help of a nonperturbative resummation technique we evaluate the spectral density, which describes the charge excitations of the system. From it, physical quantities of interest such as current and average charge can be deduced. Our main conclusions are as follows: An energy renormalization leads to a logarithmic temperature dependence of the renormalized system parameters. A finite lifetime broadening can change the classical picture drastically. It gives rise to a strong flattening of the Coulomb oscillations for low resistances, but in the Coulomb-blockade regime inelastic electron cotunneling persists. The effects become important at temperatures that are accessible in experiments.
Schoeller et al. (Thu,) studied this question.