Ultrathin single crystal buried metal layers of NiAl have been grown within (Al,Ga)As heterostructures. The layers are electrically continuous down to 1 nm in thickness. Furthermore, property changes such as increased in‐plane electron–electron interactions, and vertical transport resonant tunneling behavior, attributed to the decreased dimensionality of such ultrathin layers, demonstrate that, in this regime, the films can be considered as metallic quantum wells. The resonant tunneling has been exploited in making a three‐terminal buried metallic well device. We discuss initial results of promising experiments aimed at decreasing the lattice mismatch between the metal and the semiconductor (1) by adding In to the GaAs and (2) by substituting the more closely lattice‐matched CoAl for NiAl.