In recent years, large enterprises (e.g., Google, Alibaba, etc.) have been building and deploying their wide-area routers based on shallow-buffered switching chips. However, with legacy reactive transport (e.g., TCP Cubic), shallow buffer can easily get overwhelmed by large BDP wide-area traffic, leading to high packet losses and degraded throughput. To address it, we ask: can we design a transport to simultaneously achieve high throughput and low loss for shallow-buffered WAN?We answer this question affirmatively by employing proactive congestion control (PCC). However, two issues exist for existing PCC to work on WAN. Firstly, wide-area traffics have diverse RTTs, leading to what we called imperfect scheduling issue (e.g., data crash in time). Secondly, there is one RTT delay for credits to trigger data sending, which may degrade network performance. Therefore, we propose a novel PCC design - FlashPass. To address the first issue, FlashPass adopts sender-driven emulation process with send time calibration to avoid the data packet crash. To address the second issue, FLASHPASS enables early data transmission in the starting phase, and incorporates an over-provisioning with selective dropping mechanism for efficient credit allocation in the finishing phase. Our evaluation with production workload demonstrates that FlashPass reduces the overall flow completion times of TCP Cubic and ExpressPass by up to 32% and 11.4%, and the 99-th tail completion times of small flows by up to 49.5% and 38%, respectively.