Routing metric is very important for performance of wireless ad hoc sensor networks. ETX and ETT routing metrics cannot accurately estimate the average transmission time since they do not take back-off scheme of IEEE 802.11 in Mac layer into account. We present a new available transmission time (ATT) routing metric based on IEEE 802.11 DCF, which considers available link bandwidth and also takes physical transmission rates into account. A plugin for OLSR routing protocol daemon is implemented on Linux platform, and experiments are taken to compare ATT with ETX and ETT. Our results show that the ATT metric has the lowest packet loss rate and the lowest jitter rate among the analyzed metrics. ATT also outweigh other two metrics in the network performance, so it is the most appropriate for wireless ad hoc sensor networks.