We can locally and reversibly control the metal-insulator transition of complex-oxide heterostructures using two techniques: conductive atomic force microscope (c-AFM) lithography and ultra-low-voltage electron beam lithography  (ULV-EBL). Beginning with an insulating interface, a positive voltage applied to the c-AFM tip (or electron beam dose) can switch the interface to metallic. Conducting regions can be made with wires or dots having widths as small as 3 nm. Combining these elementary components using c-AFM or ULV-EBL enables us to create various nanostructures such as single-electron transistors (SETs), broadband THz emitters, and ballistic electron waveguides. A negatively-biased AFM probe can be used to engineer energy barriers in the conducting regions or can be used to erase the entire device altogether without affecting the reusability of the heterostructure.