  We report transport measurements in a single edge channel of an InAs/GaSb
quantum spin Hall insulator, where the conduction occurs through only one pair
of counterpropagating edge modes. By using a specific sample design involving
highly asymmetric current paths, we electrically isolate a single edge channel
of the two-dimensional topological insulator from the other edge. This enables
us to probe a single edge by multiterminal measurements. Both two-terminal and
four-terminal resistances show a nearly quantized plateau around $h/e^2$ for a
4-$\mu$m-long edge, indicating quasiballistic transport. Our approach is
advantageous in that it allows us to gain insight into a microscopic region
from local measurements.
