  We study the effect of a single bistable scatterer on conductance of
mesoscopic conductors in the crossover regime from open to closed systems.
After the resistance of the contact between the metallic grain and bulk
electrode exceeds the resistance of a grain, the effect of a bistability is
enhanced and takes the universal form. Its efficiency scales with the
cross-section of the bistable impurity and of conductances of contacts but is
independent of this impurity position inside a sample. Mutual correlations
between magneto-conductance fluctuations at two conductance levels also look
universal. Enhancement of the conductance sensitivity to variations of impurity
configurations in samples with highly resistive contacts transforms the
spectral density of a quantum excess noise toward a white-noise behavior and
requires higher stability of the sample in order to observe mesoscopic effects
in direct-current measurements.
