  Current-induced switching from metallic to insulating state is observed in
phase-separated states of (La_{1-y}Pr_y)_{0.7}Ca_{0.3}MnO_3 (y=0.7) and
Nd_{0.5}Ca_{0.5}Mn_{1-z}Cr_zO_3 (z=0.03) crystals. Application of magnetic
fields to this current-induced insulating state causes pronounced low-field
negative magnetoresistance effect [r(H)/r(0)=10^{-3} at H=1kOe]. Application of
a constant voltage also causes breakdown of ohmic relation above a threshold
voltage. At voltages higher than this threshold value, oscillations in currents
are observed. This oscillation is well reproduced by a simple model of local
switching of a percolative conduction path.
