1. A crossbar-memory system comprising: a first layer of microscale signal lines and a second layer of microscale signal lines; a first layer of one or more nanowires configured so that each first layer nanowire overlaps each first layer microscale signal line; a second layer of one or more nanowires configured so that each second layer nanowire overlaps each second layer microscale signal line and overlaps each first layer nanowire; nonlinear-tunneling resistors configured to selectively connect first layer nanowires to first layer microscale signal lines and to selectively connect second layer nanowires to second layer microscale signal lines; and nonlinear tunneling-hysteretic resistors configured to connect each first layer nanowire to each second layer nanowire at each crossbar intersection.