Fastboot doesn't recognize Nexus 5 on L
adb reboot bootloader


then 

fastboot flash &lt;whatever-it-is-you-want-to-flash&gt;


If you see  at any point when executing fastboot command, the PC is not configured to use fastboot properly. Most typically, this is because you are not running fastboot with elevated privileges. It may also be that you do not have the drivers correctly installed, which may happen under Windows 8 because of its insistence on signed drivers. - Source