Crossover between Ballistic and Diffusive Quantum Transport in Bi2Se3 Nanoribbon Devices

We study quantum interference effects in hybrid devices consisting of topological insulator Bi2Se3 nanoribbons contacted by two Al electrodes. Conductance oscillations as a function of bias and gate voltage reveal a distinct crossover in transport behavior. For junctions narrower than 500 nm, the oscillation periodicity matches the junction width, consistent with Fabry-Perot interference and quasi-ballistic transport over nearly micrometer scales. In contrast, wider devices exhibit irregular, quasi-periodic oscillations that we attribute to universal conductance fluctuations, signaling a transition to diffusive transport as the junction width becomes too large for transverse quantization to govern the interference pattern. Complementary Josephson current measurements confirm that ballistic modes persist over long distances, far beyond what magnetotransport suggests. These findings highlight the sensitivity of Fabry-Perot interference and Josephson current probes to ballistic transport channels.