  Using the density matrix renormalization group (DMRG) method, we study the
quantum coherence in one-dimensional disordered Fermi systems. We consider in
detail spinless fermions on a ring, and compare the influence of several kinds
of impurities in a gapless and a dimerized, gapped system. In the
translation-invariant system a so-called site-impurity, which can be realized
by a local potential or a change of one link, increases for repulsive
interaction and decreases for attractive interaction. The weakening of two
neighboring bonds, which is a realization of a so-called bond-impurity, on the
other hand, is healed for repulsive interaction, but enhanced for intermediate
attractive interactions. This leads to strong suppression of the quantum
coherence measured by the phase sensitivity, but not to localization. Adding a
local distortion to a dimerized system we find that even the presence of a
single site-impurity increases the metallic region found in the dimerized
model. For a strong dimerization and a high barrier, an additional sharp
maximum, even seen for systems with about 100 sites, occurs in the phase
sensitivity. A bond-impurity in the dimerized system also opens a small
metallic window in the otherwise isolating regime.
