Configure Navigation Hierarchies using Single value Choice field in SharePoint 2010 using powershell

$siteURL="http://serverName:1111/"

$site=Get-SPSite $siteURL

$web=$site.RootWeb

$list=$web.Lists.TryGetList("cl")

$field=$list.Fields["Country"]

$listNavSettings=[Microsoft.Office.DocumentManagement.MetadataNavigation.MetadataNavigationSettings]::GetMetadataNavigationSettings($list)

$mdnNavHierarchy=New-Object Microsoft.Office.DocumentManagement.MetadataNavigation.MetadataNavigationHierarchy($field)

$listNavSettings.AddConfiguredHierarchy($mdnNavHierarchy)

[Microsoft.Office.DocumentManagement.MetadataNavigation.MetadataNavigationSettings]::SetMetadataNavigationSettings($list, $listNavSettings, $true)