Try this:

string constructor2 = "16342D1F4A61BC";
File.WriteAllBytes("test.bin", ToBytesFromHexa(constructor2));

With the following helper routines:

public static byte[] ToBytesFromHexa(string text)
{
    if (text == null)
        throw new ArgumentNullException("text");

        List<byte> bytes = new List<byte>();
    bool low = false;
    byte prev = 0;

    for (int i = 0; i < text.Length ; i++)
    {
        byte b = GetHexaByte(text[i]);
        if (b == 0xFF)
            continue;

        if (low)
        {
            bytes.Add((byte)(prev * 16 + b));
        }
        else
        {
            prev = b;
        }
        low = !low;
    }
    return bytes.ToArray();
}

public static byte GetHexaByte(char c)
{
    if ((c >= '0') && (c <= '9'))
        return (byte)(c - '0');

    if ((c >= 'A') && (c <= 'F'))
        return (byte)(c - 'A' + 10);

    if ((c >= 'a') && (c <= 'f'))
        return (byte)(c - 'a' + 10);

    return 0xFF;
}