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\sect{Landauer formula for 1D conductance}
${1D}$ design with passage coefficient ${|t^{2}|}$ is attached to
electromotive force ${\varphi}$ by leads which are an ideal ${1D}$
conductors, as described in the following drawing. Show that the
conductance of the sample ${G}$,which is defined through the
equation ${I=G\varphi}$ (${I=}$ current, ${\varphi=}$electromagnetic
force) is given by the Landauer formula.
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\[G=\frac{e^{2}}{2\pi}|t^{2}|\]
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(low temperatures).
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