Single-Channel Saturation at the Quantum Conductance Limit in Single-Molecule Junctions.

Electron transport through a single quantum channel is fundamentally limited by the conductance quantum (G0 = 2e2/h ≈ 77.5 μS), achievable only in fully transparent systems without interfacial scattering. However, realizing this quantum limit in metal-molecule-metal junctions has long been hindered by intrinsic electronic mismatches at heterogeneous interfaces. Here, we report a carbon nanobelt single-molecule junction over 1 nm in length, whose conductance reaches G0, driven by the saturation of a single transport channel under ambient conditions. This unprecedented performance arises from electric-field-induced formation of covalent C-Au-C bonds at both contacts, creating atomically fused interfaces that seamlessly merge the nanobelt's π system with Au d orbitals. The resulting d-π conjugation establishes a single, transparent electronic resonance aligned with the Fermi level, suppressing backscattering and enabling near ideal quantum transport. By eliminating heterogeneous interfacial resistance at the atomic scale, this strategy offers a general blueprint for engineering atomically precise, energy-efficient nanoelectronic and optoelectronic devices.