Telecommunications stock T-Mobile US Inc (NASDAQ:TMUS) was last seen down 1.5% to trade at $207.38, following a downgrade from Oppenheimer to "perform" from "outperform." The firm cited slowing industry subscriber growth, believing the company will have a hard time beating subscriber and free cash flow estimates.TMUS has had a volatile year on the charts, recently hitting a Nov. 6 one-year low of$199.41 after trading as high as $276.49 in March. The stock's rebound from those lows was met with resistance a ...