Universal half-quantum conductance reduction by a compensated electromagnetic barrier in a quantum wire

We theoretically investigate the transport properties of a narrow quantum wire containing a Compensated Electromagnetic Barrier (CEB). Confining a CEB within a quasi-one-dimensional geometry induces a sharp Fano resonance immediately below each subband cut-off wavenumber. At the exact resonance energy, at the regime of a weak CEB, the conductance experiences a universal reduction of one full quantum unit (G₀) for every subband level, regardless of the specific parameters of the weakly coupled CEB. Furthermore, exactly at the cut-off threshold of a highly multimode wire (n≫1) and again in the regime of a weak CEB, the conductance universally drops by half a quantum unit, G₀/2. Under perfect potential compensation and weak CEB, this threshold reduction becomes independent of the barrier's specific coupling strength for any arbitrary mode number n, enabling, in a clean and noiseless environment, the detection of a quantum barrier without measuring its specific characteristics. Finally, we outline a practical experimental architecture leveraging narrow-gap semiconductors and localized nanomagnetics to resolve these transport anomalies against thermal smearing.