Self-organized nano-structures on Si substrates have drawn much current research effort. The usefulness of these structures critically depends on physical properties they can offer for potential nanoscale device applications. We report here an unusual bistable transport behavior of a tunnel junction form between an STM tip and a two dimensional (2D) Ga array self-assembled on a Si(111) substrate. At 77K, large hysteresis loops appear in the I-V spectra when electrons are injected from the tip to the 2D Ga array, characteristic of the switching between ON - OFF conductance states. The turn-on bias varies from -3.1 V to -4.0 V and shows an inverse dependence on the tip-sample distance, indicating a strong field effect. The turn-off bias, however, is essentially pinned at a conductance threshold of -2.7 V. These observations demonstrate the basis of a nanoscale tunable bistable tunnel device with the potential for digital and storage applications. @FootnoteText@ @footnote 1@I. B. Altfeder and D. M. Chen, Phys. Rev. Lett. 84, 1284 (2000).