Abstract: Each of plurality of gas sources flows to a different one of a plurality of separate source gas flow paths. Then, a source gas is distributed directly from each of plurality of separate source gas flow paths to a plurality of separate gas mixture flow paths, thereby distributing a plurality of source gases to each of different flow paths. A plurality of separate gas mixture streams is generated by flowing a plurality of source gases in each of a plurality of separate gas mixture flow paths. In some embodiments, each of a plurality of separate source gas flow paths comprises a gas distribution duct, and each of a plurality of gas mixture flow paths comprises a gas mixing conduit. In some embodiments, a gas distribution duct includes a plurality of gas distribution ports and a gas source port connectable to a gas source. In some embodiments, a gas mixing conduit comprises a plurality of gas inlet holes, a gas mixing region, and a gas outlet hole.