= Solution
The ordinary electron <Compton wavelength> is
$$
\boxed{\lambda_C=\frac{h}{m_ec}=2.4263\times10^{-12}\,\mathrm m.}
$$
A photon of that <wavelength> has <energy> equal to the electron rest <energy>, $hc/\lambda_C=m_ec^2$. It therefore sets the characteristic scale at which photon recoil cannot generally be treated nonrelativistically, and it sets the magnitude of the <Compton wavelength shift>. The reduced <Compton wavelength>, $\bar\lambda_C=\hbar/(m_ec)=\lambda_C/(2\pi)$, is a related but different convention. Localization on this reduced scale needs momenta of order $m_ec$, so a nonrelativistic single-particle description becomes inadequate. Neither wavelength is a measured geometrical radius of the electron.
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