  We present a comprehensive study of the reflection of normally incident
plasmon waves from a low-conductivity 1D junction in a 2D conductive sheet.
Rigorous analytical results are derived in the limits of wide and narrow
junctions. Two types of phenomena determine the reflectance, the cavity
resonances within the junction and the capacitive coupling between the leads.
The resonances give rise to alternating strong and weak reflection but are
vulnerable to plasmonic damping. The capacitive coupling, which is immune to
damping, induces a near perfect plasmon reflection in junctions narrower than
$1/10$ of the plasmon wavelength. Our results are important for infrared 2D
plasmonic circuits utilizing slot antennas, split gates or nanowire gates. They
are also relevant for the implementation of nanoscale terahertz detectors,
where optimal light absorption coincides with the maximal junction reflectance.
