  We analytically describe light transmission through a single subwavelength
slit in a thin perfect electric conductor screen for the incident polarization
being perpendicular to the slit, and derive simple, yet accurate, expressions
for the average electric field in the slit and the transmission efficiency. The
analytic results are consistent with full-wave numerical calculations, and
demonstrate that slits of widths ~100nm in real metals may feature non-resonant
(i.e., broadband) field enhancements of ~100 and transmission efficiency of ~10
at infrared or terahertz frequencies, with the associated metasurface-like
array of slits becoming transparent to the incident light.
