We have investigated the electron transport in AlGaAs/GaAs quantum well (QW) structures in real space of nanometer scale, using a scanning tunneling microscope (STM). The electrons were injected from the STM tip into a cleaved (110) surface of the QW structures at different distances along the [001] direction. Then by measuring the light emission intensity from individual QWs, two distinct decay constants were found. To understand the origin of the two decay constants, we developed a simulation model of hot electron transport in QW structures. From comparison between the experimental and calculated results, the two decay constants were identified with the thermalization length and the diffusion length of the injected electrons. (C) 2004 Elsevier B.V. All rights reserved.