Signal scattering in multiport electron-waveguide devices

The signal-scattering matrix $\mathbf{S}$ for multiport electron-waveguide devices or mesoscopic structures is derived from the electron-transmission probability. Like in microwave engineering, $\mathbf{S}$ relates the impinging and outgoing voltage waves, here traveling as plasma oscillations in the single-mode electron-waveguide leads. The signal switching is enhanced by low characteristic impedance in the leads. A two-port and a Y-branch-switch three-port are taken as examples. The connection between plasma-wave scattering matrices and the ac admittance of a mesoscopic structure is discussed. Explicit expressions are given for the ac-impedance matrix between the reservoirs of these structures. The phase delay stems from the finite propagation velocity of the plasma waves in the electron waveguides.