  We analyze spectral functions of mesoscopic systems with large dimensionless
conductance, which can be described by a universal Hamiltonian. We show that an
important class of spectral functions are dominated by one single state only,
which implies the existence of well-defined (i.e. infinite-lifetime)
quasiparticles. Furthermore, the dominance of a single state enables us to
calculate zero-temperature spectral functions with high accuracy using the
density-matrix renormalization group. We illustrate the use of this method by
calculating the tunneling density of states of metallic grains and the magnetic
response of mesoscopic rings.
