We study the effect of a single bistable scatterer on the conductance of mesoscopic conductors in the crossover regime between open and closed systems. After the resistance of the contact between the metallic grain and bulk electrode contacts exceeds the resistance of an individual 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 conductances of contacts but is independent of this impurity position inside a sample. Mutual correlations between magnetoconductance 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 excess quantum noise toward a white noise behavior and requires higher stability of the sample in order to observe mesoscopic effects in direct-current measurements.