  Prevailing models of resistive switching arising from electrochemical
formation of conducting filaments across solid state ionic conductors commonly
attribute the observed polarity of the voltage-biased switching to the sequence
of the active and inert electrodes confining the resistive switching memory
cell. Here we demonstrate equivalent, stable switching behavior in metallic
Ag-Ag$_{2}$S-Ag nanojunctions at room temperature. Our experimental results and
numerical simulations reveal that the polarity of the switchings is solely
determined by the geometrical asymmetry of the electrode surfaces. By the
lithographical design of a proof of principle device we demonstrate the merits
of simplified fabrication of atomic-scale, robust planar Ag$_{2}$S memory
cells.
