Use the following Powershell command to export a subsite.
Export-SPWeb "http://servername/sites/sitename" -Path "E:\Kashif\Site Backup\SampleOrder.cmp" -IncludeVersions All -includeusersecurity
Use the following to import a subsite. If your backup was broken into several files, just use the command to import the main file as shown below.
Import-SPWeb "http://servername/sites/sitename" -Path "C:\SampleOrder.cmp" -includeusersecurity -UpdateVersions overwrite