You can even take backup of the single list or document. You have to use relative URLof the document library.
Example : By default document must referenced "/Document"
There other notable parameters there IncludeVersion , IncludeSecurity and NoFileCompression.
IncludeVersions includes all the versions of the document into backup file (.cmp )
IncludeSecurity includes all the permission of the document library in the backup file.
If you used the NoFileCompression then you need to specify the folder in the Path parameter, all the contents will extracted as data file (.dat) including the XML configuration of the library and its user details,Permission and if lookup column and version and other details..
example
export-spweb -identity "http://win-k6uc8ilel11/" -path "d:\docback\" -ItemUrl "/Shared Documents/" -IncludeVersions All -NoFileCompression -IncludeUserSecurity
Here is my output on my system
Alternatively , you can also take the specific list or document library through Central Administration.
Click on Back up and Restore on the opened page , under Granular Backup click the link "Export a site or List"
Before proceeding this approach, create the network shared folder and give the write permission for the account that is being used for backup and restore.
Here is the output
 |
| Site or individual List backup through SharePoint Central Administration |
 |
| Site or List Export - through SharePoint Central Administration |
 |
| Backup Status through SharePoint Central Administration |