  Without a periodic reference framework, local structures in noncrystalline
solids are difficult to specify, but they still exert an enormous influence on
materials properties. For example, thermomechanical responses of organic and
inorganic glasses sensitively depend on the distribution of free volume or soft
spots$^{1,2}$. Meanwhile, strong electron localization$^{3}$ that endows
unparalleled electrical breakdown strengths to amorphous insulators is easily
compromised by local defects that promote inelastic tunneling over a variable
range$^{4,5}$. Here we report how metallic conduction can overcome strong
localization in amorphous insulators of small dimensions, and how local
structures can manifest their spectacular influence on such conduction. In
amorphous Si, nanoscale electrons are so coherent that they exhibit robust
quantum interferences reminiscent of the mesoscopic phenomena seen in weakly
localized metal crystals$^{6}$. Yet ultrasoft Si bonds emerge as the key local
structures whose extraordinarily strong electron-phonon interaction coerces
itinerant electrons into moving slowly at low temperature, even becoming
trapped at all temperature when Si-O/N sites are provided. The local structures
can be manipulated by a voltage or pressure to regulate charge storage, charge
flow and metal-insulator transition. Also made of Ge and oxides and nitrides,
nanostructured amorphous conductors could offer opportunities for new
applications.
