Shot noise reveals correlated multi-electron origin of the "failed insulator" state

“Failed insulators” show a negative temperature coefficient of resistivity without low temperature divergence, inconsistent with single-particle diffusion or phonon-assisted hopping. We reveal the mechanism by measurement of shot noise (SN) in tunnel junctions and ultrashort metallic junctions based on the archetypal failed insulator β-Ta. We use a combination of microwave-frequency measurements and cross-correlation noise detection technique. Metallic nanojunctions with lengths down to 8nm exhibit hot electron shot noise regime indicative of strong electron-electron interaction, with little evidence for electron-phonon scattering at T=4.2K. Meanwhile, tunnel junctions and pinhole metallic junctions exhibit substantially enhanced SN with the Fano factors clustering at even multiples of values expected for single-electron transport. We explain these observations by the combined effects of disorder and electron interactions, resulting in local Cooper pairing-like correlations among quasi-localized electrons. Our result links failed insulators to many-electron localization, and consequences for their spin-orbitronic applications.