Data are presented on a photopumped rectangular (w=40 &mgr;m,l=250 &mgr;m) quantum well heterostructure (QWH), with variable resonatorQalong the sample, showing that phonon‐assisted laser operation predominates. To achieve variable resonatorQ, the QWH sample is heat sunk embedded in In over half of its length (reflecting edges, highQ), as opposed to simple contact with a Au shim over the remaining half and no reflecting metal on the sample edges (lowQ). Photopumping at the lowQ, highQboundary near the sample center turns on and off then=1 confined‐particle recombination transition (E11) and sets a higher energy experimental (as well as calculated) reference, ℏ&ohgr;2=E11, for the lower energy phonon‐assisted laser operation, ℏ&ohgr;1=E11−ℏ&ohgr;LO.